Embodiments of the invention are directed, in general, to communication systems and, more specifically, methods of antenna switching for GNSS receivers.
As Global Navigation Satellite System (GNSS) receivers become more common, users continue to expect improved performance in increasingly difficult scenarios. GNSS receivers may process signals from one or more satellites from one or more different satellite systems. Currently existing satellite systems include global positioning system (GPS), and the global navigation satellite system (GLONASS). Systems expected to become operational in the near future include Galileo, quazi-zenith satellite system (QZSS), and Beidou.
The future road map for some wireless handsets includes having multiple antennas to support higher data rates. Often, these same handsets also include a GNSS receiver used for position navigation. The presence of multiple antennas presents an opportunity for the GNSS receiver to improve performance by leveraging the diversity gains available from multiple receive antennas.
Previous ways of solving similar problems have been presented. In GPS Receiver Satellite/Antenna Selection Algorithm for the Stanford Gravity Probe B Relativity Mission, Proceedings of the 1999 National Technical Meeting of the Institute of Navigation Jan. 25-27, 1999 Jie Li, Awele Ndili, Lisa Ward, Saps Buchman, a set of 4 antennas are assigned to 6 channels whereby in each channel 1 satellite is tracked. A master antenna is assigned for each channel for code and carrier tracking. This antenna is chosen as the one which for a given satellite either a) maximizes its SNR b) results in the highest elevation angle being computed with reference to the Satellite. So all antennas may be used in time interleaved fashion with each tracking different satellite(s). In this application, the device containing the GPS receiver is moving so the signal from each antenna is fed to each GPS channel in a time-interleaved fashion so that each channel can continue to track the same satellite signal(s) even while the attitude of the device has changed such that the best antenna for tracking a given GPS satellite signal changes.
Furthermore, several different metrics for antenna selection apart from SNR and satellite elevation are presented. In MIMO system embodiments for wireless systems, capacity based metrics or mean-squared error (MSE) type metrics are employed for antenna selection. Performance Based Receive Antenna Selection for V-BLAST Systems, IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 8, NO. 1, JANUARY 2009 Di Lu, and Daniel K. C. So
These typically involve multiple transmitted signals as well and are numerically complicated techniques which employ matrix inversion, or determinant based computations. In A Switching Circuit Scheme for a satellite site diversity system, IEEE International Symposium on Circuits and Systems, June 1988, pg 119-122, vol. 1, D. Di Zenobio, P. Lombardi, P. Migliorni, and E. Russo, switched diversity is used to improve satellite tracking by having two antennas separated by very long distances. When the performance of one antenna is suffering from rain attenuation, then the other antenna (which is hopefully not being rained on) can be used. The solution proposed above requires signal quality on both antennas to be monitored continuously.
In Predictive switched diversity for slow speed mobile terminals, M. Tarkiainen, T. Westman, Vehicular Technology Conference, May 1997, pg. 2042-2044 vol. 3, antenna diversity techniques proposed for a TDMA system with discontinuous transmissions are categorized into switching methods, selection methods and combining methods and the two switching strategies given are switch-and-stay and switch-and-examine. A switch-and-stay strategy the antenna is switched once its quality falls below a predetermined threshold, and in a switch-and-examine strategy the receiver is switched rapidly through the antennas until one with a quality above a threshold is found. The gaps in between transmissions provide convenient “quiet intervals” during which antennas can be switched without transients affecting the signal reception.
There is a need for novel antenna switching embodiments for a GNSS receiver which be classified as switch-and-stay or switch-and-examine strategies. In this case, there are no quiet intervals during which antennas may be switched (since no signal is being received). Also in this case, there are multiple signals arriving from multiple transmitters (e.g. satellites) at various different angles simultaneously.